To hang back and watch out for somebody when they are going into a potentially dangerous situation. As in before a large illegal transaction, or while conducting a robbery.

Has nothing to do with police being 5-0. Actually derived from military communications. When an infantry squad is moving, one member may be assigned to "watch the squad's 6", meaning their 6 o'clock (behind them).

Alright guys, there's supposed to be no guns inside the pool hall, so you stay out here and keep six for me. If I you hear shots or I'm not out in 20 minutes, come in blasting.